This invention relates generally to fusing systems for pressure fusing tones at elevated temperatures, and more particularly, to a method to increase the hot offset temperature of a silicone rubber fuser.
In order to affix or fuse toner material to a support surface permanently by heat, it is necessary to elevate the temperature of the toner material in order tha t he constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of support members. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bounded to the support member.
Several approaches to thermal fusing of toner images onto a support material have been described in the prior art and include providing the concomitant application of heat and pressure by a roll pair maintained in pressure contact, a flat or curved plate member in pressure contact with a roll, a felt member in pressure contact with a roll, or any other suitable means. The fusing of the toner takes place when the proper combination of heat, pressure and contact time are provided, the balancing of these parameters being well known in the art and varying according to various factors.
During operation of some fusing systems, the support member is moved through the nip formed by a roll pair. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to the fuser member is experienced under normal conditions. This is because the heat applied to the surface of the fuser member is insufficient to raise the temperature of the surface of the member above the "hot offset" temperature of the toner at which temperature the toner particles in the image areas of the toner liquefy and cause a splitting in the molten toner resulting in "hot offset." Splitting occurs when the cohesive force holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll, belt, or plate.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as offsetting, has been to provide a fuser member with an outer surface to which a release agent such as silicone oil is applied. Various polymer release materials can be used and the release agent may be applied to the fuser member by various means.
Other references of interest are U.S. Pat. No. 4,603,087 disclosing a cross-linked silicone rubber surface layer to provide improved durability in a fusing roll and U.S. Pat. No. 4,074,001 disclosing a silicone rubber surface layer on a fusing roll having a high viscosity component forming a cross-linked structure by curing and a low viscosity component effective to give releasing property to the fusing roll in order to obviate the need for an external releasing agent and to provide a long copy life fusing roll. One difficulty with the prior art methods is that the coatings often produce an uneven fuse operation. Prior art fusing methods often do not recognize the essential causes of hot offset during fusing.
It has been discovered that toner is trapped by the surface holes on the silicone rubber fuser at the hot offset temperature. To a lesser extent, surface defects such as cracks, crevasse and grinding marks also degrade the release performance. These surface holes are generally the result of tearing of the elastomer on grinding to achieve the proper roll dimensions.